Electromagnetic techniques exist to perform surveys of subterranean structures for identifying layers of interest. Examples of layers of interest in the subterranean structure include subsurface resistive bodies, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, and fresh-water aquifers. One type of electromagnetic (EM) survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic transmitter, called a “source,” is used to generate electromagnetic signals. Surveying units, called “receivers,” are deployed on a surface (such as at the sea floor or on land) within an area of interest to make measurements from which information about the subterranean structure can be derived. The receivers may include a number of sensing elements for detecting any combination of electric fields, electric currents, and/or magnetic fields.
An EM source produces an EM field by driving a controlled electrical signal that is provided through an electrode of the EM source. The signal produces an EM field that is emitted through an antenna of the EM source.
Traditionally, the electrical signal that is generated by the EM source is a switch-on/switch-off signal, where the electrical signal when in the switched-on state has a relatively large positive current amplitude, and when in the switched-off state, has either a zero or negative current amplitude. Generally, this switched-on/switched-off electrical signal is in the form of a square wave with abrupt steps (step up or step down). However, using a square wave signal to produce EM fields from an EM source limits flexibility and/or accuracy in performing EM surveying of subterranean structures.